The present invention relates to anchors or traction modules for thrust loads imparted by well tools, such as a thruster or tractor used in an assembly for performing a downhole operation in a well and more particularly to packer feet on a tractor in a bottom hole assembly, disposed on an umbilical, with a power section for rotating a bit while the tractor moves the bottom hole assembly within the well.
In the course of drilling and completing oil and gas wells, it is sometimes desirable to set an anchor in closed or open hole to serve as a reaction point for various thrust forces imparted by operating tools. Expanding anchors, very much like packers, usually are fluted around the exterior to allow flow to bypass the anchor and up the well annulus. Such externally fluted anchors will sometimes bury themselves in soft formations and completely close off all flow channels causing major well problems.
A thruster or tractor is one well tool which uses anchors as a reaction point. A tractor is part of a bottom hole assembly used on coiled tubing with the bottom hole assembly having a downhole motor providing the power to rotate a bit for drilling the borehole. The bottom hole assembly operates only in the sliding mode since the coiled tubing is not rotated at the surface like that of steel drill pipe which is rotated by a rotary table on the rig. Drilling fluids flow down the umbilical and through the bottom hole assembly and bit to cool the bit and return the cuttings up the annulus around the bottom hole assembly and umbilical to the surface. The bottom hole assembly includes a tractor which propels the bottom hole assembly down the borehole.
One such self-propelled tractor for propelling the bottom hole assembly in the borehole is manufactured by Western Well Tool and is described in U.S. Pat. No. 6,003,606, hereby incorporated herein by reference. The tractor includes an upper and lower housing with a packerfoot mounted on each end. Each housing has a hydraulic cylinder and ram for moving the propulsion system within the borehole. The tractor operates by the lower packerfoot expanding into engagement with the wall of the borehole with the ram in the lower housing extending in the cylinder to force the bit downhole. Simultaneously, the upper packerfoot contracts and moves to the other end of the upper housing. Once the ram in the lower housing completes its stroke, the upper packerfoot expands, then the hydraulic ram in the upper housing is actuated to propel the bit and motor further downhole as the lower packerfoot contracts and resets at the other end of the lower housing. This cycle is repeated to continuously move the bottom hole assembly within the borehole to drill the well. The tractor can propel the bottom hole assembly in either direction in the borehole.
The packerfoot of the Western Well Tool tractor includes an elastomeric body that inflates when filled with fluid. The elastomeric body can be made of a variety of materials such as reinforced graphite or KEVLAR®. The aft end of the packerfoot attaches to a barrel end which surrounds a cylindrical pipe on the tractor. The barrel end is slidable relative to the cylindrical pipe. The forward end is connected to the barrel end. Seals are located between the barrel end and the packerfoot and between the barrel end and the cylindrical pipe to prevent fluid escape. The packer feet include longitudinal projections or ribs circumferentially spaced around the external surface of the packerfeet so as to form flutes therebetween to provide a fluid flow area and return flow path between the ribs for the flow of returns through the annulus around the tractor during drilling. The ribs engage the earth bore which has been drilled. These longitudinal projections or ribs are not effective in soft formations because upon expansion of the packerfeet, the ribs penetrate and bury in the soft earth formation causing the flutes to become packed off with earth and closing the return flow path through the annulus for the cuttings and return fluid. Flow passages must be maintained between the packeffeet and housings to allow the passage of drilling fluids through the tractor to expand the packerfeet and to maintain the drilling. Blockage also causes the packerfeet to be blown off the tractor due to the hydraulic pressure through the annulus.
Another deficiency of prior art packerfeet is that they are made of an elastomeric, stretchable material such that upon expansion, the packerfeet balloon and stretch to engage the borehole wall. Thus when the packerfoot anchors to the borehole wall, all of the axial load and torsional load from the tractor is placed on the stretched material forming the packerfoot. These combined axial tensile loads, expansion stresses and hoop stresses are more than can be handled by a piece of fabric or elastomeric material which cannot endure these stresses. Thus it is an objective to prevent the pressure element from taking any of the torsional or axial loads from the borehole wall.
Another deficiency of the prior art packerfeet is that the amount of radial expansion is small. This is due to the limit that the reinforcing fabric which is embedded in the elastomer can expand to. An means to extend the radial expansion capabilities of packerfeet is highly desirable.
Other packerfeet are limited to expanding the packerfeet the radial distance between the propulsion system mandrel and the wall of the borehole. One design includes one wedge on each side to force a bow spring outwardly into engagement with the borehole wall. The bow springs have small rollers that are connected to the springs by axles passing through small holes in the springs. The wedges are each attached to a piston and cylinder such that when the piston moves and translates axially, the rollers ride up the two wedge surfaces so as to move radially outward and in turn push out the bow springs. Single wedges reduces the camming area for camming the packerfeet into engagement with the borehole wall creating high stresses on the carrring surfaces.
The present invention overcomes the deficiencies of the prior art.